Field of the Invention
The present invention relates to a method of repair-ing turbine blades.
Repairing turbine blades by welding is in some cases a highly satisfactory method of effecting a repair and is considerably cheaper than total replacement of the blade.
It has been found however that the repair of some turbine blades by methods using a welding process is not satisfactory since even though from visual inspection the repair appears satisfactory, problems can still arise because turbine blades are in use subjected to consider-able forces.
The type of turbine blade assembly, which may comprise a rotor or a stator, in which a repair by welding has been found to be not satisfactory is that in which adjacent blades are interconnected, usually in the region of their outer ends by snubbers, lacing wires or other members requiring a hole or other discontinuity to be formed in the turbine blade (all such members being hereinafter referred to for convenience as lacing wires and the discontinuities as lacing wire holes). The purpose of the lacing wires is to prevent excessive relative movement between the blades and to generally dampen any vibration during operation of the turbine. Such lacing wires therefore render the turbine structure more rigid but do not prevent all relative movement between the blades.
Since the outer end of the turbine blade in a rotor assembly is the part which has the greatest linear speed in use, it is the outer end that is most prone to erosion. Erosion generally occurs because of contact between the turbine blades with particles carried by the steam and also with the steam itself.
At the high pressure end of a steam turbine for example, the blades are generally subjected to damage from particles of solid matter carried by the steam due to contact between the blades and the particles, and since the rate of erosion will be dependant on relative speeds between the blades and particles, it is generally the leading edge at the outer end of each blade that is most prone to erosion by impact between the particles and said edges.
At the low pressure end of the turbine, the steam is at a lower temperature having been cooled during passage through the turbine and water condenses therefrom more readily. The steam is therefore wetter and the transfer of energy by impact on the turbine blade of the steam itself is greater at the low pressure end of the turbine than that at the high pressure end. Here again it is predominantly the outer ends of the blades which are especially subject to erosion.
It is known to form the leading edge at the outer end of the turbine blade as an insert from a different material from that used for the remainder of the blade. Turbine blades are often made from a chrome steel and the insert may be made from stellite or a suitable hardened steel, in which case the insert may be brazed to the blade. If the blade has not originally been formed with such an insert and is subsequently required to be repaired, the turbine blade may then have such an insert introduced. In which case, the insert may be secured to the remainder of the turbine blade by a welding process, the blade end being subsequently machined to restore it to its original form.
Such a method of repairing turbine blades has been found to be successful only in the case where the turbine blade is a uniform or homogeneous structure and, in the case of turbine blades having discontinuities such as lacing wire holes, then even though the welded repair may have the appearance of being satisfactory, in use such blades are prone to "stress corrosion cracking" which leads to a short and hence unacceptable life after repair.